Diviner Phabricator User Docs Configuring a File Domain

Configuring a File Domain
Phabricator User Documentation (Configuration)

Setup guide for an alternate file domain or CDN.

Overview

Serving files that users upload from the same domain that Phabricator runs on is a security risk.

In general, doing this creates a risk that users who have permission to upload files may be able to upload specially crafted files (like Flash or Java applets) which can execute with domain permissions in some contexts (usually because of security issues with Flash and Java, but both products have a rich history of security issues). The attacker can then trick another user into executing the file and gain access to their session.

The best way to mitigate this threat is to serve files from a separate domain. For example, if Phabricator is hosted at https://phabricator.example.com/, you can serve files from https://files.exampleusercontent.com/.

The alternate file domain should be a completely different domain from your primary domain, not just a different subdomain. For example, Google uses googleusercontent.com, not usercontent.google.com.

You can also configure the alternate file domain to serve through a CDN, which will improve performance.

Approaches

Broadly, you can either choose a CDN service and configure that (which will also defuse the security risks) or you can configure a second domain with the same settings as your first domain. A CDN service may be easier to set up and can improve performance.

MethodSetup DifficultyCostNotes
AWS CloudFrontVery EasyCheapRecommended
CloudFlareEasyFree/CheapRecommended
Self HostedModerateFreeNo CDN unless you're an ops wizard.

Approach: AWS CloudFront

CloudFront is a CDN service that's part of Amazon Web Services. It makes particular sense to use if you're hosting your install in AWS.

To configure it, set up a new CloudFront distribution which is pointed at your Phabricator install as an origin (make sure you point it at the primary domain name of your install, not just a load balancer or instance). You do not need to set up a new domain name, which makes setup a bit more straightforward.

Most settings can be left at their default values, but you should change the Allowed HTTP Methods setting from GET, HEAD to GET, HEAD, OPTIONS, PUT, POST, PATCH, DELETE.

Once configured, accessing the distribution's domain name should return a Phabricator error page indicating that Phabricator does not recognize the domain. If you see this page, it means you've configured things correctly.

Continue to "Configuring Phabricator", below.

Approach: CloudFlare

WARNING: You should review all your CloudFlare settings, and be very sure to turn off all JavaScript, HTML, CSS minification and optimization features, including systems like "Rocket Loader". These features will break Phabricator in strange and mysterious ways that are unpredictable. Only allow CloudFlare to cache files, and never optimize them.

CloudFlare is a general-purpose CDN service.

To set up CloudFlare, you'll need to register a second domain and go through their enrollment process to host the alternate domain on their servers. Use a CNAME record to forward a subdomain to your Phabricator install.

CloudFlare will automatically generate SSL certificates for hosted domains, which can significantly reduce the cost and complexity of setup.

Once configured, accessing the CNAME-forwarded subdomain should return a Phabricator error page indicating that Phabricator does not recognize the domain. If you see this page, it means you've configured things correctly.

Continue to "Configuring Phabricator", below.

Approach: Self Hosted

To do this, just set up a second domain exactly like your primary domain is set up. When setup is complete, visiting the domain should return a Phabricator error page indicating that Phabricator does not recognize the domain. This means that you've configured things correctly.

Note that if you use SSL (which you should), you'll also need to get a certificate for this alternate domain and configure that, too.

You can also configure a self-hosted domain to route through a caching server to provide some of the performance benefits of a CDN, but this is advanced and outside the scope of this documentation.

Continue to "Configuring Phabricator", below.

Configuring Phabricator

After you've set up a CDN or an alternate domain, configure Phabricator to recognize the domain. Run this command, providing the domain you have configured in place of the <domain> token. You should include the protocol, so an example domain might be https://cdn.phabcdn.net/.

phabricator/ $ ./bin/config set security.alternate-file-domain <domain>

Phabricator should now serve CSS, JS, images, profile pictures, and user content through the file domain. You can verify this with "View Source" or by downloading a file and checking the URL.